The purposes of this study are to 1) describe changes in plasma epinephrine (E) and norepinephrine (NE) levels, vagal tone index (VTI), flexor reflex threshold (FRT), and cardiorespiratory status following administration of morphine to premature neonates in the first 24 hours following patients ductus arteriosus (PDA) ligation, and 2) to determine if preoperative levels of E, NE, and VTI are predictive of postoperative levels of E, NE, and VTI or of changes in the levels of E, NE, and VTI, following morphine administration.